I was 23 when I started, now I'm 24.
maybe you're not realizing how underweighted I was back then. got about 13pounds in the first 2 months. maybe it's cause a few years ago I had some weight training, but I had no clue about the nutrition role in the whole thing, so it took me 6 months to get 11pounds. needless to say, I lost it all after a few months.
anyway, I know some people who were just as skinny I was, proportionally to their heights, and also got 10 to 15 lbs in the first 3 months. of course gains slowed down quite a bit after that. I see no big deal in that.